The invention relates to the field of video signal processing, and in particular interpolating a pixel from an intermediate line of a first field of a sequence of interlaced fields.
Television signals normally contain information for temporally sequential fields that are displayed on a screen, wherein two successive fields are interlaced. That is, each field contains image information only in every other image line (i.e., where an image line for which image information is present in the one field is empty in the second field).
FIG. 1 illustrates two such successive fields A and B in schematic form. The first field A includes image lines A1, A2, A3 and A4 for which a received video signal contains image information. The raster positions of fields A and B are mutually offset, such that the positions of the image lines from field B correspond exactly to the positions of the empty intermediate lines of field A, and vice versa.
In order to enhance image quality, it is often desirable to generate frames for the display of a video signal containing a sequence of fields, that is, images in which no empty intermediate lines are present.
The approach is proposed in European Patent 0192292 B1 for interpolating a pixel from an intermediate line of a field, wherein two pixels are selected from the field in which the pixel is to be interpolated. That is, one pixel of an image line above the pixel to be interpolated and one pixel of an image line below the pixel to be interpolated, with the horizontal positions of the selected pixels corresponding to the horizontal position of the pixel to be interpolated. In addition, a pixel from an image line is selected from a temporally following field, and the vertical position of the selected image line corresponds to the position of the image line of the pixel to be interpolated. The horizontal position of the selected pixel corresponds to the horizontal position of the pixel to be interpolated. In the known method, the average of the image information values—usually the brightness values or luminance values—is determined for these three pixels, and for the pixel to be interpolated the image information of the pixel whose image information value comes the closest to the average so determined is selected. The pixel of the second field is, in other words, utilized for the interpolation when the image information value of this pixel lies between the image information values from the pixels of the field with the intermediate point to be interpolated. Otherwise, the image information of one of the pixels from the adjacent line of the pixel to be interpolated is selected, with the result that one of these values is doubled (i.e., displayed twice one below the other). However, this repetition of a pixel results in a positionally incorrect display in the case of pronounced vertical transitions, a result which is particularly evident in the image along diagonal edges.
Therefore, there is a need for an improved technique of interpolating a pixel from an intermediate line of a field from a sequence of interlaced fields.